1. Field of the Invention
The present invention relates to a traction control system built into a vehicle such as a motorcycle.
2. Description of the Related Art Japanese Laid-Open Patent Application Publication No. Hei. 7-103009 discloses a conventional traction control system in a vehicle. The conventional traction control system is configured to monitor a value indicating the degree to which a drive wheel slips. The degree to which the drive wheel slips tends to be high when a vehicle is driving on a road surface (hereinafter also referred to as a bad road surface) such as a rough road surface, or a slippery road surface which is wet due to rain or the like and has a low friction coefficient. It is expected that the monitored value increases as the degree to which the drive wheel slips increases. The traction control system determines whether or not the monitored value exceeds a threshold. When it is determined that the monitored value exceeds the threshold, the traction control system initiates traction control for reducing a driving power transmitted to the drive wheel to suppress a slip.
Conventionally, in some cases, the threshold is merely a predetermined value. In other cases, also, the threshold used to determine whether or not to initiate the traction control is the same as the threshold used to determine whether or not to terminate the traction control. Even in a case where the threshold used to determine whether or not to initiate the traction control and the threshold used to determine whether or not to terminate the traction control are set to different values, the two thresholds might merely have simple hysteresis. In principle, the traction control is used to suppress a slip. Therefore, during execution of the traction control, the monitored value is a suppressed value and the monitored value changes a little with time. For this reason, it is difficult to, using the above set values, correctly distinguish between a case where a slip is mitigated after the vehicle has exited a road of a bad road surface condition and a case where the vehicle is now driving on a bad road surface and a slip is suppressed effectively by the traction control. At a time point when the vehicle has exited the road of the bad road surface condition, the traction control should be terminated immediately. On the other hand, during driving on the bad road surface, the traction control should not be terminated. According to the conventional traction control system and technique, the traction control cannot be terminated desirably.